Pall Corp., the 69-year-old Port Washington filtration company, exited the Standard & Poor's 500 Friday in advance of being acquired by Danaher Corp., leaving Long Island with just three corporations on the iconic stock index.

The move is largely a footnote to the $13.8 billion merger, which was approved last month by Pall shareholders. But it reflects a broader trend as Long Island continues to lose large public companies.

A decade ago, there were more than 100 public companies with headquarters in either Nassau or Suffolk counties. A handful of the largest were listed on the S&P 500, which tracks the performance of 500 of the most valuable companies on the New York Stock Exchange and Nasdaq.

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At its peak in 2006, Long Island had five companies on the index, used to gauge the overall stock market. They were CA Technologies, Kimco Realty Corp., North Fork Bancorp, Symbol Technologies and Pall.

But as businesses moved, folded or were acquired by corporations based elsewhere, the number of Long Island public companies traded on major exchanges has dwindled to fewer than 50. Along the way, Long Island lost three of its companies on the S&P 500.

CA moved its headquarters to Manhattan last year. North Fork Bancorp was acquired in 2008 by Capital One Bank. And Symbol was bought by Motorola Inc. in 2007.

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Danaher, which is based in Washington, D.C., expects to close its acquisition of Pall by year-end.

The Port Washington company's departure from the S&P 500 means Long Island is represented on the index by Henry Schein Inc., the medical supply giant based in Melville; Cablevision Systems Corp. of Bethpage, the cable and telecommunications company that owns Newsday; and Kimco, a real estate investment trust in New Hyde Park.

With Peter King